This project seeks to determine how obstructive sleep apnea syndrome (OSAS) is most effectively treated with nasal continuous positive airway pressure (CPAP) using Granger's (1984) model of functional assessment. Worldwide, CPAP is the nonsurgical treatment of choice for OSAS. Although it is clear that CPAP alters the physiological impairments associated with OSAS, its effects on a patient's ability to function in everyday life remain unclear, and the impact of hourly duration and frequency of CPAP use on daytime functioning is poorly understood. Previous investigations of compliance with CPAP have demonstrated variable use rates, but no study has yet evaluated the relationship between CPAP use and daytime functioning. The primary aim of this collaborative five-site project is to prospectively study a large cohort of patients varying in the severity of OSAS and sleepiness, to provide the first objective data on the relationship between hourly duration and frequency of CPAP use and improvement in daytime functioning. This project will prospectively study patients with three degrees of OSAS severity (mile, moderate, severe) and two levels of physiological sleepiness before and after three months of treatment with CPAP. A monitor containing a microprocessor will be used to objectively track CPAP usage. By assessing multidimensional aspects of daytime functioning (physiological sleep tendency, subjective sleepiness, mood, neurobehavioral performance, and functional status) we will characterize the nature of waking impairments in OSAS patients before and after treatment with CPAP. We will test the hypothesis that prior to treatment, those patients with more severe OSAS and/or greater physiological sleepiness will have the greatest deficits in daytime functioning. The project will determine the relationship between CPAP use and improvements in daytime functions relative to disease severity and level of pre-treatment sleepiness. The proposed study will provide valuable information regarding the minimal hourly duration and frequency of CPAP use necessary to produce changes in daytime functioning and to what extent these patterns of use are commonplace. Such information will contribute substantially to an understanding of the manner in which actual patient-selected patterns of CPAP use relate to therapeutically important goals, and it will identify key factors relevant to maximizing the clinical effectiveness of CPAP for OSAS.